Tree water-use efficiency and growth dynamics in response to climatic and environmental changes in a temperate forest in Beijing, China

Understanding the responses of local forests to the gradually rising atmospheric CO2 concentrations (c(a)) and changing environment is critical for appropriate management activities. This work used tree ring width measures (i.e. basal areal increment, BAI) and carbon (C) and nitrogen (N) signals to explore the intrinsic water use efficiency (iWUE) and tree growth dynamics of three major tree species (Pinus massoniana, P.tabuliformis and Larix gmelinii) in the Miyun Reservae Basin (MRB) of Beijing. The results indicate that c(a) was a primary contributor to tree growths, especially at the remote site where rising c(a) accounted for 92% and 74% of BAI changes for P. tabuliformis and L. gmelinii, respectively. N deposition was found to have a positive effect on BAI at this site. The controlling effect of c(a) on tree growth at the close-to-city site was smaller (52% and 44% of the contributions for P. tabuliformis and P. massoniana, respectively), while the negative influences of N deposition on BAI tends to be intensified. iWUE showed consistent increase during the entire growth period at all sites. Quadratic relationships between iWUE and BAI were observed, which indicated that the rising c(a) stimulated photosynthesis, contributing to the initial BAI and iWUE increase. However, the intensified water stress resulting from reduced precipitation and increased temperature led to a reduction in tree stomatal conductance causing the subsequent increase in iWUE but decrease in BAI. Of the site- and species-related responses of tree growth to c(a), climatic and environmental changes in the MRB, the site-related variation dominated. The non-linear relationship between BAI and c(a) combined with the quadratic relationship between BAI and iWUE indicate a decreased ability of forests to capture atmospheric CO2 once the CO2 tipping point has passed.